1. Move the PC.
First, try a simple solution. If your system is sitting on the desk next to you, move it onto the floor (assuming that its case is designed to sit vertically rather than horizontally). The vibration from the PC often sets up a sympathetic vibration with the desk surface, turning the case into a soundboard. Moving the computer under your desk can eliminate this problem.
You'll get the best results if the PC sits on a carpeted surface. If you have a tile or hardwood floor, try putting the system on a carpet scrap or a piece of packing foam. For a really "far out" solution, purchase keyboard, mouse, and monitor extension cables (available from any computer dealer) and move the PC farther away. This will, of course, make the CD or DVD drives harder to access.
2. Tighten up to reduce rattling.
Sometimes, the biggest contributor of PC noise isn't so much the direct sound of fans and motors as it is the vibration of PC components. It can be as annoying as a rattle somewhere in your car. You can often solve these problems by unplugging your PC, removing its cover, and methodically tightening the mounting screws of parts such as the power supply, drives, motherboard, and cooling fans. Be careful, though: Overtightening screws is worse than leaving them too loose, since you could damage components. Some noise-reduction kits include screws with polymer or rubber washers to reduce vibrations further.
3. Install gaskets.
Inexpensive polymer gasket kits can isolate vibrating fans and parts from the case, reducing noise. NoiseMagic's No-Vibes hard-drive suspension kit can do the same for your hard drives.
Follow the manufacturer's instructions.
4. Install heat-sensitive case fans.
Because of the heat generated by fast processors and other components, almost all PC cases have one or more fans dedicated to exhausting heat. The inexpensive fans that many manufacturers use can be noisy, but fans don't always have to work at full speed, since a PC produces much less heat when it's idling.
Thermostatically controlled fans use temperature sensors that slow the fans down when the internal case temperature drops.
Alternatively, some manufacturers supply single-speed fans that are designed to be quiet. Installing them is usually simple, though you may need to remove drives or add-in boards to take out the old fan and insert the new one. Note whether the fan power is connected on the motherboard or to a power-supply connector. If your new fan has an external temperature sensor, follow the manufacturer's directions to place the sensor in the optimum location.